Commit c1a834dc authored by Ingo Molnar's avatar Ingo Molnar Committed by Linus Torvalds
Browse files

timer stats: speedups 



Make timer-stats have almost zero overhead when enabled in the config but
not used.  (this way distros can enable it more easily)

Also update the documentation about overhead of timer_stats - it was
written for the first version which had a global lock and a linear list
walk based lookup ;-)

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9fcc15ec

Documentation/hrtimer/timer_stats.txt

+4 −3
Original line number Diff line number Diff line
@@ -2,9 +2,10 @@ timer_stats - timer usage statistics 
------------------------------------



timer_stats is a debugging facility to make the timer (ab)usage in a Linux

system visible to kernel and userspace developers. It is not intended for

production usage as it adds significant overhead to the (hr)timer code and the

(hr)timer data structures.

system visible to kernel and userspace developers. If enabled in the config

but not used it has almost zero runtime overhead, and a relatively small

data structure overhead. Even if collection is enabled runtime all the

locking is per-CPU and lookup is hashed.



timer_stats should be used by kernel and userspace developers to verify that

their code does not make unduly use of timers. This helps to avoid unnecessary

kernel/time/timer_stats.c

+6 −1
Original line number Diff line number Diff line
@@ -236,10 +236,15 @@ void timer_stats_update_stats(void *timer, pid_t pid, void *startf, 
	/*

	 * It doesnt matter which lock we take:

	 */

	spinlock_t *lock = &per_cpu(lookup_lock, raw_smp_processor_id());

	spinlock_t *lock;

	struct entry *entry, input;

	unsigned long flags;



	if (likely(!active))

		return;



	lock = &per_cpu(lookup_lock, raw_smp_processor_id());



	input.timer = timer;

	input.start_func = startf;

	input.expire_func = timerf;

lib/Kconfig.debug

+4 −1
Original line number Diff line number Diff line
@@ -126,7 +126,10 @@ config TIMER_STATS 
	  reprogrammed. The statistics can be read from /proc/timer_stats.

	  The statistics collection is started by writing 1 to /proc/timer_stats,

	  writing 0 stops it. This feature is useful to collect information

	  about timer usage patterns in kernel and userspace.

	  about timer usage patterns in kernel and userspace. This feature

	  is lightweight if enabled in the kernel config but not activated

	  (it defaults to deactivated on bootup and will only be activated

	  if some application like powertop activates it explicitly).



config DEBUG_SLAB

	bool "Debug slab memory allocations"

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